Methods for coating surfaces by coagulation from both acid and alkaline aqueous dispersions of polymeric particles are known in the art. Representative methods of coagulation coating from an acidic aqueous solution are discussed in U.S. Pat. Nos. 3,709,743 and 3,791,431. U.S. Pat. No. 3,791,431 discusses a method wherein an organic coating is applied to a metallic surface by immersing the surface in an acidic aqueous coating composition containing particles of an organic coating-forming material. The organic material may be in either dissolved, emulsified, or dispersed form. The coating composition is acidic as a result of the inclusion of an acidic oxidizing agent such as a mineral acid. This acidic oxidizing agent attacks the metal substrate causing metal ions to be dissolved from the surface. These ions cause the coating-forming material to be unstable in the region of the surface and, as a result, it deposits on the surface. One of the problems with this type of process is that the coating composition tends to become unstable as metal ions build up with repeated use. U.S. Pat. No. 3,791,431 seeks to remedy this problem by removing metal ions from the composition or adding a material to render the metal ions innocuous. The necessity of this additional step, of course, complicates the process and adds a further parameter which must be monitored and controlled during processing.
The process of U.S. Pat. No. 3,709,743, which is similar to the above-discussed process also employs an oxidizing acid which attacks a metallic substrate causing metal ions to form which, in turn, cause coagulation of an organic coating. Thus, this suffers the same disadvantages with respect to metalic ion build-up. The process of 3,709,743 also employs an aqueous bath containing an anionic surfactant stabilized emulsion of the synthetic resinous film-forming composition and, as a result, suffers from certain other serious deficiencies which are treated more thoroughly in the discussion of prior art alkaline bath coagulation methods set forth below. Of course, it will also be noted that both of the acidic bath embodiments disclosed in the above referenced patents are useful only to coat certain metallic substrates. It should also be noted that both of these prior art processes also are unsuitable for the application of aluminide coatings because of the presence of strong oxidizing acids.
Many prior art references disclose applying coatings such as natural latex or synthetic latices by coagulation from alkaline aqueous dispersions of essentially insoluble particles. U.S. Pat. Nos. 3,411,982 and 3,856,561 teach processes which are representative of these alkaline bath processes. These processes involve deposition of synthetic latices, which may contain small amounts of acrylic or methacrylic acid and which can be used alone or in combination with styrene, polystyrene, polyethylene chloride, polyvinyl chloride, polyvinyledene chloride and polyacrylate resins, and vinyl chloride butyl acrylate copolymers,, by polyvalent destablization of stabilized polymers. In that process the polymers are anionically stabilized or stabilized with anionic surfactants in combination with nonionic surfactants or reaction products of such. Soluble alkalies such as potassium hydroxide or ammonium hydroxide are also added in some cases to control pH and/or to assist the stabilizer in producing emulsions of the particles in water.
The presence of such anionic and nonionic surfactants or mixtures of nonionic and anionic surfactants or reaction products of such can have a deleterious effect on the final properties of coagulated polymer coatings by building up in the bath and/or in the coagulated film. Another disadvantage of such processes is the tendency of the emulsions to be unstable in the presence of chemically reactive substances such as pigments that release ions into solution and cause coagulation of dispersed film former. Still another disadvantage of such processes is that the dispersed latices have a tendency to swell in the presence of various solvents.